Note that "defending your country from invasion" isn't the survey question, its "fight a war for". For the UK, our next war is much more likely to be a war of a

Note that "defending your country from invasion" isn't the survey question, its "fight a war for". For the UK, our next war is much more likely to be a war of aggression or an intervention in someone else's conflict hence rather low desire to get involved. If there were a credible threat, I'm pretty sure the figure would be much higher. I expect the answers is highly correlated with "is the next war your country involved defending from an aggressor or being the aggressor?".